In general, a voltage regulator is a circuit that is designed to maintain a constant output voltage level as operating conditions change over time. A voltage regulator circuit provides a constant DC output voltage and contains circuitry that continuously holds the output voltage at the desired value regardless of changes in load current or input voltage, assuming that the load current and input voltage are within the specified operating range for the regulator. Maintaining accurate voltage regulation is particularly challenging when the load current variations are sudden and extreme, e.g., minimum load to maximum load demand in less than a couple hundred picoseconds. Such sudden and extreme variations in load current can occur in applications in which the circuitry being powered by the regulator is primarily CMOS logic, e.g. high performance processors. The load current presented to the regulator can change from a minimum to a maximum value very quickly when the CMOS logic switches from an idle state to a state with a high activity factor (maximum workload) due to the fact that the underlying circuitry is generally CMOS logic and hence draws only dynamic current (i.e., current that is used to charge and discharge parasitic capacitances) from the supply.
Linear voltage regulators are the most commonly used types of voltage regulators in integrated circuits (ICs) and have a number of advantages. Linear voltage regulators are fully integrable, requiring no off-chip components such as inductors. Unlike switching types, linear regulators generate no inherent ripple of their own, so they can produce a very “clean” DC output voltage, achieving low noise levels with minimal overhead (cost). The output voltage correction in linear regulators is achieved with a feedback loop; however; some type of compensation is required to assure loop stability. The need to maintain adequate loop stability, also referred to as “phase margin,” limits the achievable bandwidth of linear regulators. Therefore, any linear regulator requires a finite amount of time to correct the output voltage after a change in load current demand. This “time lag” defines the characteristic called load response time (TR), which may not be fast enough for applications with sudden and extreme load current variations.
To overcome slow response time as well as relatively low power efficiency of high bandwidth linear regulators, a “bang-bang” type voltage regulator can be used. The fast response time makes bang-bang type voltage regulators more suitable than their linear counterparts to handle highly varying load current demands with minimal effect on regulated voltage, as they are capable of providing nearly instantaneous response to any variation in load current demand. In general, a bang-bang voltage regulator utilizes a passgate device (e.g., PFET or NFET) which is switchably operated to fully turn “on” and “off” to supply/sink current (header/footer) and achieve fast response time to load changes. The fast response time also improves the high-frequency power-supply rejection ratio (PSRR).
The use of bang-bang regulators, however, poses a major design challenge with regard to limiting the intrinsically generated ripple on the regulated output that results from the sudden switching of the current of the passgate device (bang-bang operation). The passgate which is controlled in a bang-bang fashion has to be sized to handle the weakest corner (e.g., with minimum drain-to-source voltage (Vds) across the passgate) to guarantee regulation, but such a passgate will be too strong (in other words, oversized) for other corners (e.g. with maximum Vds). This results in increased intrinsic ripple amplitude, which is not a desirable behavior in bang-bang type regulators.